Electrical termination systems are commonly used for providing electrical termination to electrical signals present in electrical circuits. Typically, an electrical termination system attempts to provide a predefined impedance or load, such as a fifty ohm impedance, between a signal line and a reference line, such as a ground reference, in an electrical circuit. Ideally, a termination system should provide a constant predefined impedance for a wide range of electrical signal frequencies. Examples of termination systems include, but are not limited to, radio frequency (RF) loads for frequency circulators, couplers, power combiners, absorptive filters, and antenna replacement dummy loads.
Typically, terminations are present in RF & microwave systems to absorb unwanted signals which may otherwise cause component instabilities, distortion products, or component damage. High power terminations were historically large planar resistors exhibiting poor RF impedance due to large parasitic shunt capacitance. The introduction of internally matched stripline devices offered designers drop-in wide band low VSWR with no need for external matching. While a great step forward in performance, internally matched terminations require more attention to the design of the launch from the electrical circuit to the termination to achieve rated performance. Clever designers found that although these termination devices were internally matched, their performance could be further optimized over a narrow range of frequencies with external matching. Being a stripline component, the termination's performance may also be adversely affected by the proximity of other components or enclosure walls and covers. As wireless systems demand isolators with greater isolation and couplers with greater directivity, a better solution is required.
As an example, a radio frequency (RF) signal is terminated via a fifty ohm impedance to ground by electrically coupling a fifty ohm resistor between an RF signal line and a ground reference line in the electrical circuit. The fifty ohm resistor, in this example, is electrically coupled between the signal line and the ground line using a length of coaxial cable. The coaxial cable includes an electrical connector to mate with an electrical connector electrically coupled with the electrical circuit thereby making electrical interconnection of the fifty ohm impedance with the electrical circuit a relatively quick process for a technician to perform.
Such exemplary electrical termination systems as discussed above traditionally have been enclosed in a housing of aluminum or copper. The housing provides an internal cavity and a plane therein to which a ceramic fifty ohm resistor is soldered and used to terminate an RF signal coupled thereto via a coaxial cable. The ceramic fifty ohm resistor termination can be directly contacting the conductor of the coaxial cable or, alternatively, the resistor may be electrically coupled to the conductor of the coaxial cable via an electrical coupling structure, such as an electrical circuit disposed on a sheet metal arrangement, to electrically couple the ceramic fifty ohm resistor termination with the conductor of the coaxial cable.
To provide a fifty ohm termination to the electrical circuit as described above, a high degree of care must be taken in designing and making the electrical termination system. The termination system must be designed and constructed to closely match the electrical transition via the coaxial cable from the electrical circuit to the termination resistor to a fifty ohm transition to reduce reflective energy coming back out of the electrical termination system into the electrical circuit to be terminated. This matching must be effective for all the relevant signal frequencies that would be present at the signal line and that require a fifty ohm termination to ground.
The fifty ohm termination resistor is normally enclosed in the cavity of the metal housing to electrically seal up, as an electrical shield, the termination resistor in the cavity and to protect the termination resistor from the elements such as dust.
Unfortunately, the termination system described above requires significant labor and technical craftsmanship to get a fairly good fifty ohm transformation via the coaxial cable into the RF resistor. The sad fact is that most such termination systems have resulted in electrical mismatching of the termination to a corresponding electrical circuit, increased reflection of energy from the termination, and very specific operation only within a narrow bandwidth of frequencies for an electrical signal. Furthermore, each high-power RF resistor used for such termination normally has associated with it unwanted electrical parasitics, such as excess capacitance and inductance, which make it very difficult to design a standard termination system that matches an electrical circuit while trying to get an electrical signal over a wide band of frequencies into the fifty ohm resistor and then down to ground.
To deal with this problem, a conventional termination system normally requires tuning of the termination circuit for each termination system to try to get an effective matching circuit. During manufacture and assembly of such a termination system, a technician typically has to tune each electrical circuit termination to try to get low reflections out of each such termination. This tuning typically involves changing electrical circuit components in the termination circuit and/or varying a length of a coaxial cable for interconnecting the termination system to an electrical circuit. This regrettably requires a lot of manual labor to make each termination system, and typically specified for operation in a narrow range of frequencies for a particular application for the termination system.
Additionally, this tuning adds significant burden to a technician when assembling together the coaxial cable with the termination device. It requires the technician to be very careful about the placement of the circuit elements you put together and the amount of solder used in solder joints and things of that nature. And when done tuning the termination, the device is typically specific to a narrow range of frequencies for an application. In other words, it is not a generic device that would plug into various electrical circuit designs. It typically would be useful only for one electrical circuit design. In general, tuning is done on each individual termination system that is produced, not by design. This adds significant cost to mass manufacturing of termination systems due to 1) the recurring technician labor for tuning each termination system, and 2) the requirement to stock different types of termination systems for different narrow bands of frequencies and applications. In addition to being very careful how each termination system is tuned during manufacturing, customers have to be very careful about the length of the coaxial cable used with the termination system. Essentially, the fact that the termination system is tuned to a very narrow band of frequencies also gives a significant ripple or change in magnitude of reflected energy over frequency and the length of the coaxial cable must be carefully selected to minimize energy reflection into the customer's electrical circuit at a certain frequency band. Unfortunately, the length of the coaxial cable becomes very critical in the operation of the termination system. Conventional termination systems, consequently, have been very ad hoc and customized solutions on a per application basis.
Thus, there exists a need for a termination system that does not have the aforementioned disadvantages of the prior art. Particularly, it would be very desirable for a termination system to 1) have low reflections of energy across a range of frequencies, 2) not require technician tuning for each individual termination system, 3) work over many different applications for many different systems and not have to be customized for each specific application, and 4) achieve desired performance irrespective of the coaxial cable length used to electrically couple the termination system to a specific electrical circuit for an application.